1. Field of the Invention
The present invention relates to ignition systems for motor vehicles.
2. Description of the Related Art
In an ignition system for a motor vehicle, electrical energy is typically converted from low voltage (typically 12 volts) to high voltage for the firing of the spark plugs of the engine of the vehicle. The conversion normally takes place through an ignition coil assembly having a primary coil and a secondary coil, the coils magnetically coupled to one another. A controller causes electrical current to increase to a predetermined level at a relatively slow rate in the primary coil. Once at the predetermined level, the controller abruptly breaks the current. The result is a large voltage induced in the secondary coil. This large voltage causes a spark across the gap of a spark plug electrically connected to the secondary coil. This spark ignites an air-fuel mixture in the engine.
A problem can be encountered with an ignition system operating as just described. This problem can occur if the spark plug has a very small gap. In this event, the current building up in the primary coils which induces a (relatively small) voltage in the secondary coil, can induce enough voltage to cause a spark across the gap of the spark plug. This spark, totally unintended, may cause pre-ignition in the engine.
One possible solution to this problem is the addition of a diode in series between the secondary coil and the spark plug. This diode takes advantage of the fact that the unintended spark (and attendant current flow between the secondary coil and the spark gap) is in the opposite direction from the intended spark produced by the ignition coil assembly. The diode will block the unintended current flow which may otherwise tend to occur.
Although this solution may be effective, the usual way in which it is normally applied has at least one limitation. Generally, such a diode is incorporated in such a manner that leaving the diode off for applications in which it is not needed (i.e., applications with relatively normal spark gap sizes) is not practical. The result is that for all applications in which the ignition coil is used, the diode is included. Obviously, this is wasteful in the case of applications where the diode is not needed.
Therefore, an ignition coil design which easily allows a diode to be incorporated as needed will provide advantages over alternative solutions.